Holmes, 26, is charged with multiple counts of first-degree murder and attempted murder for opening fire inside a suburban Denver cinema during a late screening of the Batman film "The Dark Knight Rises," in July 2012.

The mass shooting left 12 people dead and 70 wounded or otherwise injured in the melee. Holmes has pleaded not guilty by reason of insanity, and prosecutors said they will seek the death penalty if he is convicted.

The hearing in Arapahoe County District Court on Friday centers on proposed testimony by crime scene experts who prosecutors intend to call at trial.

Defense lawyers object to crime scene reconstruction testimony through expert witnesses, on the grounds that it is unreliable, and on Friday cross-examined FBI forensic examiner Brett Mills over his findings.

Mills conceded he was not trained on the equipment he used to measure bullet trajectories, which he used as part of his analysis.

The proceeding follows four days of closed-door hearings held this week on whether Holmes should undergo a further sanity examination.

Defense lawyers have conceded that Holmes was the sole gunman, but have said he was in the midst of a "psychotic episode" when he went on the shooting spree.

They also said in court filings that Holmes suffers from a "chronic and serious mental illness" and should not face the death penalty should a jury convict him.

After invoking the insanity plea, the former neuroscience graduate student underwent a court-ordered sanity examination last summer. Conclusions reached by evaluators have not been made public, but in November prosecutors sought to have Holmes undergo an additional evaluation by their experts.

The murder trial had been set to begin next month, but was postponed indefinitely by Arapahoe County District Court Judge Carlos Samour after prosecutors filed the request.

In ordering this week's sanity hearings closed to the public, Samour said dissemination of testimony about the sanity evaluations would jeopardize Holmes' right to a fair trial.

"The information that will be discussed at this hearing goes to the heart of the case," Samour said.

It is unknown when he will rule on the prosecution request. (Editing by Dan Whitcomb and Gunna Dickson)